In order to perform power supply or perform charging, wireless power transfer techniques have been gaining attention. Research and development are being conducted regarding a wireless power transfer system wirelessly performing power transfer to various electronic apparatuses such as mobile terminals and notebook computers and household electrical appliances or to power infrastructure equipment.
Note that, in general, a strong coupling wireless power transfer technology uses four procedures of electromagnetic induction, magnetic field resonance, electric field induction, and electric field resonance.
Conventionally, techniques using electromagnetic induction, and techniques using radio waves are known as this kind of wireless power transfer, and for example, the wireless power transfer using electromagnetic induction has been used to transmit power to various consumer electronics such as a shaver or an electric toothbrush.
On the other hand, recently, there have been raised expectations for power transfer techniques using magnetic field resonance and electric field resonance as techniques being capable of transferring power to a plurality of power receivers while placing each power receiver at a certain distance from a power source, or to various three-dimensional positions of each power receiver.
In a wireless power transfer system using resonance phenomena, a high efficiency power transfer may be possible, even though distances or sizes of respective power sources and power receivers. Further, when a power receiver is located at a position within an area of overlapping power transfer areas (power transfer ranges) of a plurality of power sources, a high efficiency of wirelessly transferring power may be realized for various postured power receivers by optimizing intensities and phases of the respective power sources.
As described above, power transfer suitable for a position or posture of the power receiver may be performed by controlling intensities and phases of outputs of a plurality of power sources wherein power transfer ranges of the power sources are overlapped.
However, when the number of power sources is plural, calculations for obtaining phases and intensities of the power sources may become complicated. Specifically, when a power receiver is located at a position within an area of overlapping power transfer ranges of a plurality of power sources, so as to perform an optimized power transfer, the plurality of power sources may be controlled, and therefore, a processing time for calculating the phases and intensities becomes long and resources for performing calculations becomes large.
Therefore, in the wireless power transfer system including a plurality of power sources and a plurality of power receivers where power transfer ranges are overlapped, there are some fears that power transfer may not performed in an optimum state. Further, for example, practical realizations of a wireless power transfer system using magnetic field resonance or electric field resonance may be delayed.
A variety of wireless power transfer techniques have conventionally been proposed.    Patent Document 1: Japanese Laid-open Patent Publication No. 2011-199975    Patent Document 2: Japanese Laid-open Patent Publication No. 2008-283789    Non-Patent Document 1: UCHIDA Akiyoshi, et al., “Phase and Intensity Control of Multiple Coil Currents in Resonant Magnetic Coupling,” IMWS-IWPT2012, THU-C-1, pp. 53-56, May 10-11, 2012    Non-Patent Document 2: ISHIZAKI Toshio, et al., “3-D Free-Access WPT System for Charging Movable Terminals,” IMWS-IWPT2012, FRI-H-1, pp. 219-222, May 10-11, 2012